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Atoms, Ions, and Molecules

Models of Molecules

Each molecular model has its own advantages and disadvantages. Molecular models provide information about the type and number of atoms found in a molecule and some models show molecule shape and bond types.

A molecular model shows the elements and the number of atoms of each element present in the molecule. Two examples of molecular models are the ball-and-stick and space-filling models. Scientists can use other models to depict molecule shape and bond types, such as the valence-shell electron-pair-repulsion (VSEPR) model. A structural formula is a formula that shows how atoms are arranged in a molecule and the types of bonds between the atoms. In the structural formula for water (H2O), the oxygen atom has two hydrogen atoms attached via single bonds and two pairs of unshared electrons also attached. Unshared electrons are typically not shown in structural formulas.

In a structural formula, elements are represented by their symbols, and bonds are represented with lines. A single bond is represented by a single line, a double bond is represented as a double line, and a triple bond is a triple line.

Structural Formulas of Various Molecules

Water (H2O) and methane (CH4) are examples of molecules that have only single bonds. Carbon dioxide is an example of a molecule that has double bonds. Acetylene has a triple bond as well as single bonds.
A drawback of structural models is that they cannot show three-dimensional molecular geometry. For example, in methane, the four hydrogen atoms are arranged around the carbon atom in a three-dimensional tetrahedral shape. A basic structural formula cannot convey this three-dimensional shape. A perspective drawing is a variation of a structural formula that indicates a three-dimensional view. In a perspective drawing, a line indicates a bond that lies on the surface plane. A wedge, or solid triangular bond line, indicates a chemical bond that is coming out of the page, toward the viewer. A dashed wedge indicates a bond that is away from the viewer, directed behind the surface plane.

Perspective Drawing of Methane

In this perspective drawing of methane (CH4), the solid wedge indicates that a hydrogen (H) atom is bonded to the front of the carbon (C) atom (toward the viewer). The dashed wedge indicates that another hydrogen atom is bonded behind the plane of the carbon atom (away from the viewer). The two solid lines indicate that two hydrogen atoms are bonded to the carbon atom in the plane of the carbon atom.
Two other three-dimensional models are also often used in chemistry to represent molecules. A ball-and-stick model is a representation of a molecule that uses spheres to represent atoms and bars to represent bonds. A space-filling model is a representation of a molecule that uses spheres to indicate overlapping atoms. In both of these models, different elements are shown by spheres of different colors. The symbol of the element is sometimes printed on the sphere. The sizes of the spheres reflect differences among the size of the atoms that make up the molecules.

Ball-and-Stick and Space-Filling Models of Methane

The ball-and-stick model of methane shows the four single bonds between each hydrogen atom, shown in white, and the central carbon atom, shown in black. The same colors are used in the space-filling model, but the bonds are not explicitly shown. In both models, the spheres used to represent hydrogen atoms are smaller than the sphere used to represent the carbon atom because the radius of the bonded hydrogen atom is smaller than the radius of the bonded carbon atom.
Both ball-and-stick and space-filling models show how atoms are bonded in a molecule. In this regard, they are similar to structural formulas. In ball-and-stick models, double and triple bonds can be shown by double or triple bars, respectively. Furthermore, the angle between two bonds can be clearly shown in a ball-and-stick model.

A space-filling model shows the shape of the entire molecule, which is sometimes useful in visualizing the function of a molecule, as it shows three-dimensional arrangement as well as size relationships between atoms. It determines the possible closeness of approach of groups to one another and the degree of crowding of atoms in various arrangements. For example, a space-filling model indicates intense molecular crowding, as between the bromines in 1,8-dibromonaphthalene.